We are performing extensive dynamic 31P exercise studies in the BLSA population too develop bioenergetic correlates of aging, walking speed, and other outcome measures from the BLSA. Aerobic fitness and muscle bioenergetic capacity decline with age; whether such declines explain age-related slowing of walking speed is unclear. We hypothesized that muscle energetics and aerobic capacity are independent correlates of walking speed in simple and challenging performance tests and that they account for the observed age-related decline in walking speed in these same tests. Muscle bioenergetics was assessed as postexercise recovery rate of phosphocreatine (PCr), kPCr, using phosphorus magnetic resonance spectroscopy (31P-MRS) in 126 participants (53 men) of the Baltimore Longitudinal Study of Aging aged 2691 years (mean = 72 years). Four walking tasks were administeredusual pace over 6 m and 150 seconds and fast pace over 6 m and 400 m. Separately, aerobic fitness was assessed as peak oxygen consumption (peak VO2) using a graded treadmill test. We found that all gait speeds, kPCr, and peak VO2 were lower with older age. Independent of age, sex, height, and weight, both kPCr and peak VO2 were positively and significantly associated with fast pace and long distance walking but only peak VO2 and not kPCr was significantly associated with usual gait speed over 6 m. Both kPCr and peak VO2 substantially attenuated the association between age and gait speed for all but the least stressful walking task of 6 m at usual pace. Overall, muscle bioenergetics assessed using 31P-MRS is highly correlated with walking speed and partially explains age-related poorer performance in fast and long walking tasks.